In the prior art of MEMS technology relating to the provision of sealed cavities housing functional components such as mechanical structures (e.g. inertial sensors, accelerometers, timing devices such as oscillators and gyros to mention a few), it has previously been difficult or even impossible to control the atmosphere in the cavities. The sealing of any vent holes or other openings in the structure has necessarily been performed using processes where the atmosphere is conditional on the process. This means that the sealing takes place either in vacuum, whereby the cavities have been sealed with a vacuum prevailing therein, or in some other atmosphere required by the process used (e.g. oxygen), and thus cavity atmosphere cannot be controlled at will.
However, often times it is desirable or even absolutely necessary to control the atmosphere in terms of pressure and/or gas composition, and in such circumstances the prior art methods fail. An example of the latter is a high performance mechanical structure that requires damping of some kind, preferably gas damping with a controlled pressure in order to avoid ringing or self-oscillation.
One solution to this problem has been devised by Stemme et al in published International Patent Application WO 2011/073393A2. The inventors therein provide a material having a larger dimension than a hole in a substrate to be sealed. A tool is used to press the material into the hole to seal it. Optionally the material is heated to melt it whereby the pressing action is made easier. This disclosure relates to the processing of flat surfaces exhibiting no topology.
In U.S. Pat. No. 4,744,863 (Guckel) discloses sealed cavity structures suitable for use as pressure transducers formed on a single surface of a semiconductor substrate (20) by, for example, deposition of a polycrystalline silicon layer. A cavity structure comprising channels is made. The channels extend from communication with the atmosphere to the cavity. The cavity may be sealed off from the external atmosphere by a second vapor deposition of polysilicon or silicon nitride, which fills up and seals off the channels, or by exposing the substrate and the structure thereon to an oxidizing ambient which results in growth of silicon dioxide in the channels sufficient to seal off the channels.